When raw water is filtered using a permselective membrane module directly or after pretreatment, a pressurized or submerged type module is generally employed. With the pressurized type, permselective membranes are housed in a vessel, and raw water is fed to the membranes by means of external pressure produced by, for example a pump, and filtered. With the submerged type of module, permselective membranes are submerged in a filtration tank, and raw water is sucked through the membranes to obtain the filtrate.
Pressurized hollow fiber membrane modules are classified into outside-in and inside-out types. With the outside-in type, raw water is fed to the outer surface of the hollow fiber membranes and filtered from the inside of the membranes. With the inside-out type, raw water is fed to the inner surface of the membranes and filtered from the outside of the membranes. After a certain period of filtration, cleaning is generally carried out to remove suspended solids deposited on the surface of the hollow fiber membranes. More particularly, with the outside-in type, part of the filtrate is forced to flow from the inside of the membranes to the outside, and with the inside-out type, from the outside to the inside, thereby peeling the solids off of the membrane surface.
Because the outer surface area of the membrane is larger than the inner surface area, the outside-in type, in which raw water is supplied from the outside, reduces the amount of water per membrane area to decrease the load on the membrane, which is advantageous for producing the same amount of filtrate. In recent years, however, hollow fiber membrane modules have increased in size, and the number of hollow fiber membranes contained in the vessel has also increased. Thus, although suspended solids on the surface of the membranes near the center of the membrane bundles can be removed by cleaning, it is difficult to efficiently discharge the solids to the outside of the bundles, causing the possibility of re-deposition on the membrane surface.
An air scrubbing technique is used in the cleaning described above. In air scrubbing, air is introduced from the lower part of a hollow fiber membrane module. The resulting bubbles rise through membranes and shake the hollow fiber membranes, thereby widening the gap between membranes and efficiently discharging the suspended solids to the outside of the module. In a large module, however, even if air is introduced from the outer periphery of the bundles, the air does not reach the hollow fiber membranes near the center of the membrane bundles. This results in problems such as the inability to clean suspended solids on the surface of hollow fiber membranes near the center of the membrane bundles.
Various approaches have been taken to overcome these problems. For example, in the module shown in FIG. 1 of Patent Document 1, which comprises U-shaped bundles of hollow fiber membranes bonded thereto, when air is introduced from below the U-shaped region, the bubbles pass between membranes to reach the membranes near the center of the bundles, thus allowing efficient cleaning. However, because the hollow fiber membranes in the U-shaped region are not fixed, the membranes are strongly shaken during air scrubbing, resulting in possible membrane breakage. Moreover, during this shaking, friction is produced between the membranes and the binding member that is used to form the U-shape, resulting in possible membrane breakage.
Patent Document 1: JP H10-28846 A
Patent Document 2 suggests providing a feed water inlet through a space opened in a resin partition which seals hollow fiber membranes with resin. The space for the feed water inlet is provided by binding a plastic tubular material in parallel with hollow fiber membranes, and then pulling off the tubular material after curing the resin. This requires the additional task of pulling off the tubular material, and may also cause poor sealing if the resin for sealing the membranes in contact with the tubular material is removed together when pulling off the tubular material. Moreover, if a resin with high rigidity, such as epoxy resin, is used, the resin may crack while removing the tubular material. In addition, it is very difficult to locate the tubular material in a predetermined position by binding it with hollow fiber membranes. When inserting hollow fiber membranes with the tubular material being fixed at a predetermined position, the tubular material must be avoided while inserting the membranes, resulting in possible breakage of the membranes in contact with the tubular material.
Patent Document 2: JP S61-242607 A
Patent Document 3 discloses a hollow fiber membrane module comprising a number of porous hollow fiber membranes around a pipe with perforations in the side surface. In the module, at least one of the pipe and hollow fiber membrane bundles is housed in an outer cylindrical vessel while being fixed with an adhesive. This pipe, however, is equipped with a raw water inlet, an outlet for draining the liquid inside the vessel, and a cleaning air inlet for vibrating the hollow fiber membranes. This configuration requires opening and closing the valves, as well as switching flow paths for each filtration and cleaning cycle, thus complicating the operation of a filtration apparatus composed of this hollow fiber membrane module. Moreover, the location of the air release valve on the side surface of the vessel results in incomplete discharge of air from the module. This incomplete discharge of air further increases the possibility of suspended solids adsorbing onto the hollow fiber membranes.
Patent Document 3: JP H7-136469 A
Patent Document 4 discloses a method for manufacturing a hollow fiber membrane module. The method comprises inserting hollow fiber membrane bundles into a casing, attaching a temporary sealing cap to each end of the casing in the longitudinal direction of the casing, fixing only the ends of the membrane bundles by curing a resin inside the temporary caps, sealing the casing ends and membrane ends by adhesive curing, and removing the temporary sealing caps in the longitudinal direction of the casing.
Patent Document 4: JP H7-31852 A
Patent Document 5 discloses a hollow fiber membrane separating module. In this module, a number of hollow fiber membranes contained in the module casing are divided into two to seven bundles, each wound with a protective net. The ends of each bundle wound with a protective net are fixed to the module casing with an adhesive, while being separated from the ends of other bundles with a partition plate therebetween. Thus, in this module, one bundle is contained in one section. Whereas this kind of configuration is applicable to relatively small modules or modules with sections of specific shapes, it is difficult to apply to large modules or modules with sections of complicated shapes (shapes with relatively many acute angles).
Patent Document 5: U.S. Pat. No. 2,932,394
Patent Document 5 also discloses the technique of winding a protective net around bundles of hollow fiber membranes and accommodating the bundles in the module casing. Similarly, Patent Document 6 discloses the technique of preparing a module by binding hollow fiber membrane bundles with a tubular protective net and loading the bundles into a casing.
Patent Document 6: JP H2-203924 A
Patent Document 7 discloses a hollow fiber membrane filtration module having one end of the hollow fiber membranes sealed and the other end open. In this module, a tubular material with perforations in the side surface near the center of hollow fiber membrane bundles is fixed with an adhesive to the hollow fiber membrane bundles, with the end thereof opposite to the open end of the membranes being open. The aforementioned Patent Document 3 also discloses a hollow fiber membrane module comprising a pipe with perforations in the side surface, the pipe being used as a raw water inlet, an outlet for draining the liquid inside the vessel, and a cleaning air inlet. Both the tubular material and the pipe described in these patent documents serve as a raw water inlet.
Patent Document 7: JP S61-192310 A
Patent Document 8 discloses a hollow fiber membrane module containing hollow fiber membrane bundles provided with a fixed end, achieved by fixing both ends of a number of hollow fiber membranes with an adhesive. In this module, slits for introducing air are provided in the fixed end of the membrane bundles at the lower end of the module. Because the fixed end of the hollow fiber membrane bundles is shaped with an adhesive resin, the slits in the fixed end are made after the adhesive resin is cured, or by curing the adhesive together with a member with slits, and then pulling off the member. Providing slits by these methods poses problems such as the resin cracking during processing, damage to the hollow fiber membranes, and unstable product quality.
Patent Document 8: JP S61-106307 U